The attachment of two members can be made in a number of different ways. Some attachment methods such as adhesives, glues, or welding can form a strong bond between the members without generating any squeezing or clamping force between the members. Fasteners such as a typical rivet or tack pins/screws, also do not create a clamping force on the members. When a clamping force is desired, screws are a common method of achieving this.
Many consumer product assemblies are being designed with a thin-walled enclosure that is softer than the fasteners being used to assemble them. This coincidence works in favor of a self-clinching fastener which requires a hardness differential, and clinches at the surface of the panel. This is opposite for a screw however, and limits the available clamp load of the screw. In this case the limiting force for clamp load is equal to the yield strength of the threads engaged by the screw in the enclosure. This invention applies to this type of assembly.
Because complete stripping of the thread would render an assembly useless, the yield force is used for design purposes to calculate the maximum allowable induced load. The yield force of the female threads is calculated as the yield stress of the thread in the bottom panel as shown in FIG. 1, multiplied by the area of the thread that is in shear. The area of the thread in shear is the cylindrical surface per thread that would be left if a screw were to cut the threads out if pulled or pushed vertically. Because the effect of the thread helix on this area is very small, the area of thread in shear can be simplified to be the number of threads in shear multiplied by the cylindrical area of one thread.
In FIG. 1, the clamp load of the prior art is generated when the screw is tightened. Tightening the screw causes a tension force in the screw. That is to say the part of the screw between the underside of the head and the top of the engaging threaded member. Part of the clamp load generated stretches the screw like a spring. Hooke's Law describes the change in length of the screw. The shorter a screw is made, the less capable it is of stretching. Hooke's Law applies equally for the top panel, which is compressed by the clamp load. The force experienced by the top sheet is equal to the clamp load. In the case shown in FIG. 1, the panel is thin which also tends to limit the potential compression. It is the application of a hard screw into a thin soft enclosure that enables a clinch attachment to compete very well against a screw thread.
Referring now to FIG. 2 another example of a prior art fastener is depicted. Since a tack pin or a tack screw is pressed in, and is not in tension during installation, it can provide little or no clamp load. The compression of the top sheet can provide a component of clamp, however the situation is slightly different from the case of a screw. When the installation load is applied to the tack, and consequently the top sheet, both will change in length (shorten or compress) per Hooke's law. As long as the parts were not crushed beyond their yield points, both will relax (attempt to return to their original lengths) when the installation load is removed. We can remove force and express relative deflection in terms of known dimensions and material properties. Assuming homogenous properties of the bottom sheet, three different scenarios exist for pin compression compared to panel compression. Given our small part case, we can make the following statements: If the tack and panel have the same deflection, then they will spring back nearly equally after the punch is removed, and no clamp load would be generated. If the tack has a greater stiffness, and hence smaller deflection than the panel. Panel compression can happen, and consequently a small clamp load, given that the tack will spring back less than the panel tries to. If the panel has a greater stiffness, and hence smaller deflection than the tack, then no clamp will be generated. The tack might actually spring back higher than the attaching panel, leaving a small gap under the head. An example of where a tack pin/screw would create a small clamp load is where the top sheet might be a gasket material like rubber. As with the screw, the yielding of the undercut in the bottom sheet is the overriding maximum clamp load allowable for a tack pin/screw if the top sheet has the capability of generating such a force.
Clinching Tack pins/screws are an excellent alternative to very small screws, and provide numerous advantages over a screw, including:                1) No tapping of hole is required        2) Thread stripping as a failure is eliminated        3) Tack pins/screws are pressed in, as such installation is greatly simplified        4) Tack pins/screws do not require a thread locking patch and are inherently non-loosening        5) Tack pins/screws have very thin heads and permit thinner design forms than screws which require thicker heads due to the drivers required in the head.However, some shortcomings of Tack Pins/Screws are:        1) Little or no clamp load (while it is minimal, a short screw does provide a bit of clamp load)        2) If under installed, a gap will be left under the head permitting axial float        3) Perfect installation, where the bottom of the head of the tack pin contacts the top sheet during installation, without over pressing, requires expensive equipment.Simply adding a separate washer to the screw/pin of the assembly creates all of the problems associated with a greater number of parts and Belleville washers cannot be made small enough to be used with a micro screw or very small tack pins having a diameter in the range of 1.0 mm. There is therefore a need in the art for a new type of unitary fastener which when applied can provide a residual clamp load to the elements it has joined.        